This invention relates to an electrical circuit for automatically adjusting the gain of an amplifier used to drive an analog meter so that the meter automatically reads at mid-range. A preferred use of the present circuit is with equipment for use in the fiber optic industry and in particular with equipment for aligning the ends of optical fibers when the ends of the fibers are to be joined as by splicing.
The fiber optic industry has been rapidly developing in recent years to accommodate such new technologies as high speed communication circuits. Concomitant with this rapid development has been an increasing need to have the ability to properly splice optical fibers when a break occurs in a fiber for one reason or another.
When two ends of optical fibers are to be spliced together, it is necessary to align the ends of the fibers before splicing to maximize light transmission across the splice junction. A conventional method of aligning optical fibers prior to splicing is to inject light into one of the optical fibers, towards the end to be spliced and then to measure the light transmitted across the junction with a detector positioned to detect the light received in the second fiber. In conventional apparatus, the light detected in the second fiber is converted into an electrical signal which is amplified and directed to an analog meter. With this apparatus the fiber ends are moved relative to one another until the signal, which is visually displayed by the meter, is at a maximum. When this occurs the fibers are optimally positioned for splicing.
Unless the person doing the splicing has access to the two distant ends of the two fibers being spliced, there is a problem of getting light into and out of the fiber which is necessary for the detection apparatus to operate. In principle, by bending a fiber, light energy can be injected into the core of an optical fiber. Conversely, by bending a fiber, light energy within the core will escape. It is this property of optical fibers that makes possible "local injection - detection instruments", as they are called. These instruments aid in splicing optical fibers by allowing an operator to manipulate the ends of the fibers being spliced while measuring the amount of light being transmitted through the splice rather than working through others who make measurements remote from the break.
With conventional apparatus, there are problems in scaling the meter used to display the electrical signal representative of the light transmitted across the splicing junction. These difficulties arise because the fibers are constructed of different materials and use different cladding. There are also different splicing methods used. As a result, it has been found that the ratio of the light injected into the fiber and the signal detected on the other side of the junction may vary from 20,000 to 1. With present day instruments it is not uncommon to have offscale readings on the meter because of the significant variations in the light energy transmitted across the junction when the optical fibers are misaligned or different types of fibers are being spliced.
Techniques in existence for coping with this wide range of signal strengths include manual gain control circuits and automatic ranging circuits which divide the total dynamic range into discrete increments in order to obtain signals with enough resolution to properly align the connection. With these circuits, the gain of the measuring circuit needs to be continually adjusted in order for the meter to remain near the center of the scale.